Systems of the type referred to, in which the remote transponder units are serially interconnected along the transmission line, are vulnerable to line faults which may occur anywhere on the line. Although the presence of an open line fault or a ground fault may readily be apparent, the fault may be difficult to identify and locate, especially in the case of a long line. Moreover, until the fault has been rectified the signalling capability of the system will generally be seriously reduced, presenting a serious problem where a high degree of security is required, as in a burglar alarm or a fire alarm system.
It is an object of the present invention to provide a multiplex signalling system for the two-way transmission of data over a single transmission line, having means for automatically isolating a line fault while maintaining full signalling capability while the fault exists.
It is another object of the invention to provide in such a system means whereby the fault may readily be located from the control station.